On the basis of experimental observations described earlier, we have proposed that marginal regeneration is caused by surface tension gradients at the borders of mobile foam films, Marginal regeneration is the rate-determining mechanism in the drainage of such films, and, as such, a determining factor in the persistence (or long-term stability) of foams. Marangoni flows are caused by surface tension gradients, and these set off the exchange of thicker for thin film elements along the borders, while the total film area remains the same. In this paper we present simulations of the drainage of Liquid in a vertical soap film, and show that it is realistic to expect large surface tension gradients along the lower border of the film under the conditions which lead to marginal regeneration,